The goal of the proposed study is to evaluate the efficacy and safety of switching to Vilazodone in patients with major depressive disorder (MDD) who are unresponsive to, only partially responsive to, or cannot tolerate a trial of the generic SSRI, citalopram (e.g., "partially responsive" means patients who report that their depressive symptoms have improved through the use of citalopram but that significant depressive symptoms persist; "cannot tolerate" refers to patient report of intolerable side effects that result in a desire to discontinue the medication). Seventy-two subjects with major depressive disorder who are still symptomatic or report intolerable side effects after a 6-week open-label trial of citalopram 20mg/day ( i.e. who are not classified as responders) will be randomized to receive a higher maximum dose of citalopram (40mg/day) or switch to vilazodone during the randomization phase of the trial for 6 weeks. The hypothesis to be tested is that vilazodone will result in greater rates of treatment response and be better tolerated compared to being titrated up to a higher maximum dose (40mg/day) of citalopram. The proposed study will provide needed data on the efficacy of switching antidepressants when individuals do not fully respond to previous treatment or have intolerable side effects with a generic SSRI.